The transmission force or torque transmission capability of continuously variable double-cone type transmissions is determined, to some extent, by the engagement pressure between the lateral faces of a transmission belt and the engaging conical faces of the cone disks. It is known to utilize hydraulic pressure to engage a movable cone disk against one side of the transmission belt chain, or other transmission medium, for short and generically, hereinafter "belt", the other side of the belt then engaging against a facing conical side of the other one of the conical disks of the pair, which acts as a counter element. To provide hydraulic pressure, a control valve is connected to a cylinder-piston arrangement which acts axially with respect to the movable conical disk. A torque sensor is interposed in the driving shaft to sense the transmitted torque and, in dependence thereon, permits drainage of the pressure fluid or blockage thereof, or drainage under throttled conditions, in accordance with transmitted torque, so that the engagement pressure of the disks is maintained at a predetermined level. The torque sensor also permits control of valves to add a predetermined volume of hydraulic pressure fluid into the system from a hydraulic pump.
U.S. Pat. No. 4,261,213, to which German 28 28 347 corresponds, describes such a continuously variable transmission system in detail, and also describes a suitable torque sensor. As well known, a transmission of this type can be constructed with torque sensors located both on the driving shaft as well as on the driven shaft. This is particularly suitable if the characteristic of the shafts is changeable, which, for example, may occur when a dynamo electric machine drives a first shaft, and a dynamo electric machine is coupled to the second shaft which, for example when current is disconnected from the machine driving the first shaft, can act as a generator for dynamic braking, or to feed back mechanical energy to the dynamo electric machine on the first shaft, for generation of electrical energy, for example recharging of a battery. The operation of the respective shafts, thus, can change, and either shaft can be a driving shaft or a driven shaft.
Infinitely variable transmissions are sensitive to slippage of the belt with respect to the cone disks. Slippage may occur, for example, upon abrupt changes in loading. To prevent slippage, the torque sensor, as known, reacts to change in torque by increasing the supply pressure available. This is done by throttling drainage from the hydraulic system, which is a closed loop, until the basic pressure of the hydraulic system corresponds to the value required to transfer the torque applied to the driving shaft. The system is so arranged that, upon rapid or abrupt change in loading, the torque sensor not only completely closes drainage or return flow from the system to a pump but, additionally, pumps a certain predetermined volume of pressure fluid back into the hydraulic loop system.
The torque sensor, as known, is placed in that portion of the loop of the hydraulic which is in the path of the return flow of the hydraulic fluid. Thus, if the torque sensor is to increase the quantity of fluid which is in the pressurized portion of the loop, it must do so via the hydraulic control valve. The hydraulic control valve, usually, is an axially shiftable spool valve. Such spool valves inherently include throttling regions so that the response time of the hydraulic pressure fluid system is unduly long upon abrupt changes in applied torque. Further, long connecting lines of the pressure medium and a plurality of flow resistance regions and throttling locations cause a delay in the build-up of pressure upon abrupt changes in torque. The system requires the placement of throttling gaps or diaphragm holes in the supply lines; further, the construction of the spool valve itself, and particularly of a four-edge type spool valve, introduces throttling points which retard the build-up of pressure upon change in torque. Upon sensing of a sudden change or abrupt rise in torque, which may occur for example in a vehicular variable transmission upon commanded change of speed, the quantity or volume of pressurized fluid and available from the torque sensor to enhance the pressure in the pressure portion of the system must flow through the control valve and the supply lines from the control valve to the cylinder-piston arrangements of the conical disks, which also means through the various throttling positions and flow resistance connections as well as the long flow lines. As a result, the required volume of pressurized fluid, and the volume of pressurized fluid supplied by the torque sensor, reaches the cylinder-piston arrangement of the movable disk only with some delay with respect to the increase in torque.
The effect of the delay can be expressed as high throttling of hydraulic fluid between the torque sensor and the cylinder-piston arrangement, in effect the decrease of application pressure between the conical disks. In a transmission, and for example in an automotive transmission, there then will be gap of compressive force between the conical disks acting on the belt, which may lead to slippage of the belt with respect to facing conical surfaces of the disks. Slippage of the belt may have serious consequences, since the belts, which may have metal reinforcements, may cause irreparable damage to the disks, or may become irreparably damaged themselves. Usually, slippage may result in localized evaporation of lubricants, resulting in local overheating at incremental areas of the belt and/or the conical faces where the belt and the conical faces contact each other. This, then, changes the design parameters of the frictional engagement between belt and conical disks and may cause, particularly at the conical disks, localized changes in hardness of the disks, or loss of surface hardening, resulting in loss of resistance to wear and formation of grooves in the faces of the disks--which should be smooth.